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Recent laboratory efforts and telescopic observations of Europa have shownthe relevance of a yellow colouration of sodium chloride (NaCl) caused bycrystal defects generated by irradiation. We further investigate this processby irradiating (with energetic electrons) different types of analogues whereNaCl is associated in different ways to water ice. We produce two types of icyanalogues: compact slabs and granular particles where we investigate twoparticle sizes (5 and 70 $\mu$m). We perform electron irradiation at cryogenictemperatures (100 K) and under high vacuum (10-7 mbar) conditions, withenergies of 1 and 5 keV. We observe the formation of two different types ofcolour centres. The so-called F-centres (460 nm) were formed in every sample,but the intensity of the absorption band within the compact slabs surpassed anyother icy analogues and was comparable to the intensity of the absorption bandwithin pure NaCl grains. M-centres (720 nm) have not been detected at thesurface of Europa so far, and were close to the detection limit during ourirradiation of compact slabs. The slabs could be good analogues for Europa_ssurface as they produce mainly F-centres. Other notable differences have beenobserved between compact slabs and granular samples, such as the presence of anabsorption band at 580 nm attributed to colloids of Na, exclusively withingranular samples. Such absorptions have not been reported in previous studies.

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VIS spectroscopy of NaCl-water ice mixtures irradiated with 1 and 5 keV  electrons under Europa_s conditions: Formation of colour centres and Na  colloids